1. Field of the Invention
The present invention relates to a conductive layer connection structure of a semiconductor device electrically connecting an upper conductive layer and a lower conductive layer, and a method of manufacturing thereof. More particularly, it relates to a manufacturing method of a conductive layer connection structure of a semiconductor device having a natural oxide film removed using a titanium silicide layer, and a conductive layer connection structure formed using the method thereof.
2. Description of the Background Art
Sputtering and CVD (Chemical Vapor Deposition) are well known as film deposition techniques. Sputtering has an advantage that a film is easily obtained without the need of adjusting the gas flow and temperature as in the case using CVD. A method of forming an upper conductive layer using sputtering will be described hereinafter.
Referring to FIG. 17, an interlayer insulation film 3 is formed on a lower conductive layer 5. A through-hole 9 is formed in interlayer insulation film 3 reaching lower conductive layer 5. By colliding Ar ions with an aluminum plate 1, the aluminum atoms fall downwards due to the collusion. This process is continued to result in the state shown in FIG. 19 via the state of FIG. 18. Reference number 7 indicates an upper conductive layer of aluminum.
However, the aluminum atoms do not fall down perpendicularly as shown in FIG. 17 in practice. The fall of the aluminum atoms caused by the collusion of Ar ions is seen in various directions as shown in FIG. 20. An aluminum film is not easily formed at the corner 10 of through-hole 9.
The opening dimension of through-hole 9 has become smaller in accordance with the size of devices becoming smaller. The thickness of interlayer insulation film 3 is substantially fixed to a constant value considering the possibility of a pin hole. Therefore, the aspect ratio (hole depth/opening size of hole) of the through-hole is inevitably increased. A higher aspect ratio makes it further difficult for the aluminum atoms to reach corner 10 of through-hole 9. This will induce problems that will be described hereinafter.
FIG. 21 is a sectional view of a semiconductor device where an upper conductive layer 7 of aluminum is being formed on interlayer insulation film 3 having through-hole 9 of a high aspect ratio. FIG. 22 shows the state of the semiconductor device after the formation of upper conductive layer 7. It can be seen that the opening of through-hole 9 is clogged up with aluminum before the internal of through-hole 9 is fully filled with aluminum, resulting in a void 11 in through-hole 9. Void 11 will cause a higher electrical resistance in the aluminum film in through-hole 9. This results in a greater possibility of electromigration at this portion. Electromigration is a phenomenon of metal atoms being moved when a great current stress is applied to the metal. If the metal atoms travel in a direction opposite to that of the current, there will be no aluminum at the cathode side to create a void, whereas aluminum gathers at the anode side to generate hillocks and whiskers. The defect caused by electromigration includes increase in interconnection resistance and disconnection on account of voids and short circuit between multilayer interconnections on account of hillocks and whiskers.
Therefore, an upper conductive layer is formed using a CVD method when the aspect ratio of a through-hole is great. According to the CVD method, a film is formed by the gas affecting the film formation face. Gas can easily diffuse into the corner of a through-hole. This means that the internal of a through-hole can be completely filled up even if the through-hole has a great aspect ratio.
A method of filling up a through-hole with a conductive layer by a CVD method will be described hereinafter. This method is disclosed in, for example, 1990 IEEE Jun. 12-13, 1990 VMIC Conference pp. 219-225 "CONTACT HOLE FILL WITH LOW TEMPERATURE LPCVD TiN" Ivo J. Raaijmakers et al.
Referring to FIG. 23, interlayer insulation film 19 is selectively etched to form a through-hole 21 reaching to impurity region 17. Reference number 13 indicates a silicon substrate, and reference number 15 a field oxide film. Referring to FIG. 24, a natural oxide film 23 is formed on the exposed impurity region 17 by the oxygen in the atmosphere. Because the presence of natural oxide film 23 will degrade the electrical connection between impurity region 17 and a TiN film that will be formed afterwards, natural oxide film 23 is reduced as follows.
Referring to FIG. 25, a Ti film 25 is formed all over the main surface of silicon substrate 13 by sputtering.
Referring to FIG. 26, silicon substrate 13 is subjected to heat treatment in a nitrogen atmosphere at a temperature of 650.degree. C. for 30 seconds. The portion of the Ti film in contact with interlayer insulation film 19 becomes a TiN (O) film 29. TiN (O) film 29 is a film having oxygen dispersed in a TiN film.
Regarding the portion of the Ti film in contact with impurity region 17, Ti diffuses into impurity region 17 to be bonded to Si in impurity region 17 to result in TiSi.sub.x 27 (0&lt;x&lt;2). Because TiSi.sub.x is reductive, a portion of TiSi.sub.x reacts with 0 in the natural oxide film to become TiSiO. Thus, the natural oxide film is reduced.
Referring to FIG. 27, TiN film 31 is formed all over the main surface of silicon substrate 13 by a CVD method. Through-hole 21 can be completely filled up with TiN film 31 even if the aspect ratio of through-hole 21 is great because the formation is carried out by a CVD method.
Referring to FIG. 28, Al-Cu film 33 is formed on TiN film 31. Al-Cu film 33 serves to improve the conductivity of the interconnection layer.
In the above-described conventional method, the Si used in the formation of TiSi.sub.x 27 is supplied from impurity region 17. If the reaction between Ti and Si is carried out excessively, TiSi.sub.x 27 will break through impurity region 17 as shown in FIG. 29 to damage the pn junction. Thus, leakage of current occurs.
The thickness of upper interconnection layer is great because of its three layered structure of Al-Cu film 33, TiN film 31, and TiN (O) film 29. This will result in a greater stepped portion in the layer above the upper layer interconnection layer with problems such as a possibility of disconnection in an interconnection layer above the the upper layer interconnection layer.